Conventional wisdom holds that electrical motors and generators are mature and that their efficiency can improve only incrementally. The truth is that these machines operate with 90–99% efficiency near their rated speed and load but with much lower efficiency when the speed or load varies from the rated values.
In a conventional electric motor, the maximum rotor speed and torque required by the application determine the key design parameters. The motor typically performs optimally only in a narrow speed-torque band centered close to the maximum steady-state speed and torque. Performance drops off when the motor must supply an intermittent burst, when the motor needs to deliver only a small fraction of its rated torque or power, or when it runs at low speed. Indeed, the specification sheets for power motors typically do not specify torque-efficiency performance for rotor speeds below about 1500 RPM, since motor performance degrades precipitously at those speeds.
Consequently, a diverse range of motor designs has evolved in the art, with each design optimized to serve a narrow set of applications. For example, a three-phase AC motor can drive a conveyor belt in a factory with close to 99% efficiency if the belt starts up once per shift and carries a nearly constant load. In practice, however, many applications require frequent starts and stops and encounter a wide range of loads. Therefore, a motor's efficiency rarely remains consistently high under real world operating conditions.
Use of electric motors to power electric vehicles (EVs) became somewhat more practical when engineers began replacing DC motors (which usually require brushes to make electrical contact with the spinning rotor) with three-phase AC motors. Since then, efforts have focused on improving the inverter that converts stored DC power into three-phase AC power to drive the motor. The inverter must also vary the frequency of the three-phase power to control the motor speed effectively. The Partnership for a Next Generation of Vehicles, a joint R&D effort between government and industry, has identified the inverter as a key technical obstacle to commercializing EVs at a reasonable price. The preferred motor design of the present invention eliminates the need for an external inverter—the inverter is integrated into individual coil modules.
In a conventional three-phase induction motor (3PIM), three sinusoidally varying currents 120° apart drive three electromagnets that comprise the stator, the part of the motor that remains fixed in place. Together these electromagnets create a strong primary magnetic field whose direction spins like the hour hand of a clock. This rotating field induces currents in the windings of the rotor (the part of the motor that spins) that in turn create a reactive magnetic field, which tries to align itself with the primary field by twisting the rotor. The rotor spins continuously, lagging behind the stator field. Each motor operates most efficiently at some design-specific combination of speed and torque that optimizes the rate at which the rotor slips a full rotation behind the stator field (the “slip rate”). Lower speeds induce a smaller secondary field and hence generate lower torque, in addition, at lower speeds the rotor slips more frequently, reducing energy transfer to the rotor and increasing dissipation (loss of energy as heat). A 3PIM becomes an adjustable speed drive motor with the addition of an inverter, an expensive power module that generates three-phase AC power with adjustable frequency and amplitude from a DC supply to vary the speed of the motor. Reducing the frequency allows the motor to start up more gently and efficiently but also reduces the torque supplied. In general, varying the amplitude and frequency of the power provides some flexibility to tune motor operation, but cannot overcome the limitations inherent in the 3PIM's fixed number of poles and the geometric factors that determine the ideal angular velocity of the rotor relative to the stator field precession.